Efficient AC/DC rectification of low voltages is important for the realization of fully-functional vibrational energy harvesting systems. Microwatt to milliwatt motional/vibrational energy harvesting systems can utilize low-power power management circuits. Vibrational energy harvesters can utilize ac/dc converter circuits that can operate at low input power and low voltage with acceptable efficiency (>80%). Attention has been given to circuits for piezoelectric harvesters [Ottman, G. K., Hofmann, H. F., Bhatt, A. C. and Lesieutre, G. A., 2002, Adaptive piezoelectric energy harvesting circuit for wireless remote power supply, IEEE Trans. Power Electronics, 17, 669-76; Guan, M. J. and Liao, W. H., 2007, On the efficiencies of piezoelectric energy harvesting circuits towards storage device voltages, Smart Mater. Struct., 16, 498-505; Lefeuvre, E., Audigier, D., Richard, C., Guyomar, D. K. 2007, Buck-Boost converter for sensorless power optimization of piezoelectric energy harvester, IEEE Trans. Power Electronics, 22, 2018-25], whose output voltage level is generally higher (typically >1 V) than similarly sized magnetic harvesters (typically <1 V). Accordingly, there is a lack of suitable low-voltage rectification solutions for magnetically based vibrational energy harvesting systems [Mitcheson, P. D., Yeatman, E. M., Rao, G. K., Holmes, A. S., and Green, T. C. 2008, Energy harvesting from human and machine motion for wireless electronic devices, Proceedings of the IEEE, 96, 1457-86].
Passive junction-based semiconductor diode bridges are generally not suitable for input voltage levels under 0.5 V, due to the forward-bias voltage drop associated with the diodes. Active MOS-based synchronized rectification is possible, where the rectification is implemented by controlling the conduction of MOSFET with a clock signal that is in phase with the input signal [Ghovanloo, M. and Najafi, K. 2004, Fully integrated wideband high-current rectifiers for inductively powered devices, IEEE J. Solid-State Circuits, 39, 1976-84]. Active MOS-based synchronized rectification typically utilizes a drive voltage (control signal) that is higher than the threshold voltage of the MOSFET (typically ˜0.4 V). In order to make available this drive voltage, complicated driving circuitry is commonly utilized.
Alternatively, in an attempt to avoid the inherent forward-bias voltage drop of semiconductor diodes, active diodes have been used in wireless power transmission [Lam, Y-H, Ki, W-H and Tsui, C-Y, 2006, Integrated low-loss CMOS active rectifier for wirelessly powered devices, IEEE Trans. Circuits Systems-II: Express Briefs, 53, 1378-82] and medical areas [Lehmann, T. and Moghe, Y., 2005, On-chip active power rectifiers for biomedical applications, IEEE Intl. Symp. Circuits Sys. ISCAS, 2005, 732-5]. Active diode can refer to a comparator-controlled switch that replaces junction-based diode. Recently, a combination of a synchronized rectifier and an active diode for energy harvester application has been investigated [Peters, C., Spreemann, D., Ortmanns, M. and Manoli, Y. 2008, A CMOS integrated voltage and power efficient AC/DC converter for energy harvesting applications, J. Micromech. Microeng., 18, 104005-13]. However, this approach can suffer from the same voltage threshold limit of other self-driven synchronized rectifiers, and the minimum rectifiable input voltage was reported in the Peters et al. reference to be only 1.25 V.
Accordingly, there is a need in the art for a method and apparatus for high efficiency AC/DC conversion of low voltage inputs.